A hybrid internal combustion-electric powertrain may include a plurality of electric motors in combination with an internal combustion engine. A first or primary electric motor may be used in series or alternatively in parallel with the internal combustion engine to provide power to either the front wheels or rear wheels of a vehicle. A supplemental drive system may be employed to provide power to the wheels not getting power from the internal combustion engine. The supplemental system may include a drive unit for each wheel, with each unit further including an electric motor and a gear set disposed between the electric motor and the associated wheel. Known supplemental drive units may have a limited operating cycle time period. It is desirable to provide a supplemental electric drive system with drive units suited for operating for sustained periods of time that may be accommodated within an available space of a vehicle.